1. Field of the Invention
The invention relates to a method and apparatus for arranging balls onto a mounting object, and more particularly to a method and apparatus for arranging solder balls as ball bumps onto electrodes on semiconductor chips, electrodes on IC-chip-mounted substrates, or electrodes of semiconductor devices.
2. Description of the Related Art
As junction media between electrodes of a semiconductor chip and a circuit board, fine balls (bumps) are used. Besides, techniques of transferring fine balls onto semiconductor chips, substrates of IC chips, or the like to form bumps in a ball grid array (BGA) have been increasing their importance. Here, such semiconductor chips and substrates of IC chips to have the balls transferred on are referred to collectively as mounting objects. As the bumps, solder balls and gold balls are used. In the cases of using solder balls as the bumps, the solder balls are arranged on the surface of a mounting object at positions where the bumps are to be formed, and then heat is applied to deposit the solder balls to the mounting object. To carry out the deposition, flux or a solder paste (hereinafter, referred to collectively as "flux") needs to be applied between the mounting object and the respective solder balls.
To simultaneously transfer a number of balls onto a mounting object, a technique is known in which the balls are held by suction on a ball arrangement device before the balls are transferred from the ball arrangement device to the mounting object. The ball arrangement device has ball suction holes for holding the balls by suction. The positions of the respective ball suction holes correspond to positions over the mounting object onto which the balls are to be transferred. The balls are held by suction on all the ball suction holes in advance, and the ball arrangement device holding the balls by suction is then conveyed to a mounting base. On the mounting base, the mounting object to receive transferred balls is placed in advance. The balls can be transferred by pressing the balls held by suction on the ball arrangement device against the surface of the mounting object being placed on the mounting base.
In the cases of using solder balls, flux needs to be applied between the mounting object and the respective balls. Methods for applying flux include a method of applying the flux onto the surface of the mounting object at the positions to receive the transferred balls in advance of the transfer of the balls, and a method of applying the flux onto the surfaces of the balls held on the ball arrangement device.
As a first method of applying the flux onto the surface of the mounting object, a flux printing method by using a mask is known. As the mask, a metal mask or a mesh mask is used. The mask has opening portions, which are provided at positions corresponding to the positions on the surface of the mounting object where the flux is to be applied. The mounting object is covered with the mask, the flux is applied from above the mask, and then the mask is removed to obtain the mounting object having flux transcribed on predetermined portions.
The above-described method of using a mask, which is a method in wide use conventionally, has regularity in the amount of transcribed flux. The method, however, has drawbacks in that flux to be supplied to the opening portions of the mask involves bubbles, and in that excessive flux gets into a backside of the mask (the side being in contact with the mounting object) to pollute the backside of the mask, thereby the surface of the mounting object is polluted with the flux. Once such troubles occur, it is also difficult to clean up only the backside of the mask. In the method of using a mask, no additional problem occurs as long as the surface of a mounting object is flat; however, in the cases where the surface of a mounting object is uneven, it becomes impossible to put a mask into close contact with the surface, so that the transfer of flux will not be carried out normally.
As a second method of applying flux onto the surface of a mounting object, a method is known in which pins corresponding in number to the number of the balls to be transferred are arranged like a pinholder, flux is applied onto the extremities of the pins, and then the respective extremities of the pins are brought into contact with the mounting object so that the flux is transferred from the extremities of the pins to the mounting object for application.
In the above-described method of using pins, flux adhering and remaining on the extremities of the pins after the transfer causes a fluctuation in the amount of flux to be transcribed onto the extremities of the pins at the next transfer, giving rise to a problem of deteriorating the stability in the amount of transcribed flux. Besides, another problem consists in that a rise in the number of solder balls to be mounted on a mounting object increases the complexity and the cost of the pin device.
As the method of transcribing flux onto the surfaces of the balls held on the ball arrangement device, a method is known in which flux is applied on a flat plate and thinly spread to a prescribed thickness with a squeegee or the like before the balls on the ball arrangement device are brought into contact with the flux application film on the flat plate to transcribe the flux onto the surfaces of the balls. Thereafter, the balls are transferred from the ball arrangement device onto a mounting object. As a result, the flux is transcribed between the balls and the mounting object to mount the balls.
In the above-described method of transcribing flux onto the surfaces of the balls held on the ball arrangement device, when flux is spread and applied over the flat plate with a squeegee or the like having some flux adhering thereon, the resultant applied flux becomes uneven in thickness. The unevenness in the thickness of the flux on the flat plate is negligible when the balls are great in diameter. However, if the balls are small in diameter, the unevenness in the thickness of the flux produces a problem in that the flux is transcribed onto not only the surfaces of the balls but the surface of the ball arrangement device. If some flux is transcribed on the surface of the ball arrangement device, redundant balls may get adhered on the flux-transcribed portions of the ball arrangement device during the next operation of holding balls, leading to an additional trouble in polluting balls in a tray. On that account, it has been impossible to transcribe flux onto balls below 150 .mu.m in diameter by this method.
Besides, in the above-described method, the transcription of flux onto balls is repeated over the flux applied and spread on a flat plate. As a result, the flux on the flat plate is exposed to the surrounding atmosphere over a long period of time, which gives rise to a problem in that the flux is dried to suffer a change in viscosity.
Moreover, in the above-described method, when spreading flux over the flat plate by using a squeegee or the like, some flux is pushed aside from the squeegee to accumulate on all sides of the flat plate. Here, in order to prevent the ball arrangement device from being polluted with the accumulated flux, the flat plate needs to be provided excessively large in dimension. Therefore, on the occasions of periodical cleaning of machines, the peripheries of a flux transcription unit around the flat plate take a long time to clean up, causing a reduction in the operating time of the unit.
In Japanese patent publication (non-examined) No.10058135, method and equipment for applying flux is disclosed. The method is to apply the flux to a plurality of external pads (electrodes) of semiconductor devices. In that method, a flux applying equipment is provided, having an application plate in which a plurality of application holes are opened. Each application holes is opposite to each said electrodes, and the flux is pressed out of the application holes so as to be applied to the electrodes.
When flux 8 becomes uneven in its amount to be pressed out of the application holes, excessive flux can run out of some application holes while other application holes are not filled with the flux up to the surfaces since they are so small in the amount of flux, possibly resulting in that the flux is not uniformly filled into all the application holes. The occurrence of such situation hampers the stable transcription of the flux onto the respective electrodes. In the cases where the number of electrodes of semiconductor devices is as great as several tens to several thousands to push up the number of electrodes to have flux simultaneously transcribed on, it becomes impossible to fill the flux uniformly into the respective application holes.